Bushy,
Re your distinction between the work of Chippendale and Mahon - I suggest they were both right, because they were looking for different things. Chippendale was correct in stating that the actions of the pilots placed the aircraft in a situation where it suffered CFIT - and that is CFIT - it was not a person in Auckland who disconnected the A/P and descended the a/c to 1600'. So he was looking at the actual actions in the immediate lead-up to the impact. That's understandable, he was a copper.
However Mahon then looked at why the crew thought it was ok to act as they did - that is, what they (as competent, experienced aviators) were thinking - and he found that there was background to it which did stretch back to Auckland.
One was looking at 'what has happened'? - an a/c hit a mountain; the other was looking at 'what caused the a/c to be in a position to hit the mountain?' - human factors and the ol' chain...